Abstract:

Objective: In order to provide theoretical basis for revealing drying mechanisms and precise control of wood platen drying process,the effects of heating platens' temperature on temperature and pressure inside poplar lumber with moisture content (MC) beyond fiber saturation point (FSP) were studied,the moisture state variety in surface layer and center layer of poplar lumber was further analyzed,and the moisture migration mechanisms were explained accordingly. Method: The dimension of poplar lumber specimen was 400 mm (L) ×120 mm (T) ×25 mm (R). The wood initial MC range was 40%-60%. The platen drying experiments were conducted under three levels of heating platens' temperatures,i.e. 120,130 and 140℃. The temperature and pressure in the surface and center layer of lumber was measured with temperature-pressure integrated sensor in drying process,and the moisture state was further analyzed in accordance with the thermodynamics principle,which is based on the relationships between saturated pressure and saturated temperature of vapor. Result: 1) The peak values of temperature and pressure inside poplar lumber were increased with the increase of heating platens' temperature. As the heating platens' temperature increased from 120℃ to 140℃,the peak values of temperature and pressure in the surface layer of lumber were increased from 115.4℃,209.3 kPa to 133.0℃,314.5 kPa,respectively. The peak values of temperature and pressure in the center layer of poplar lumber were increased from 109.4℃,235.5 kPa to 126.4℃,336.9 kPa,separately. However,the peak temperatures' occurrence time lagged behind the related peak pressures. 2) The analysis for moisture state in the surface layer of poplar lumber showed that the moisture in poplar lumber was unsaturated water at the heating platens' temperature of 120℃. The final MC of surface layer,which was higher than FSP,provided the evidence for the conclusion. The heat transferred from the heating platens to surface layer of poplar lumber resulted in the large volume vaporization of free water with the heating platens' temperature of 130 and 140℃. As drying continues,state of vapor in the surface layer of poplar lumber was changed from the saturated state to superheated state,and the final MC of surface layer was lower than FSP,which provided evidence for the conclusion. Moisture state analysis for the center layer of poplar lumber showed that the moisture in lumber was unsaturated water with the MC beyond FSP. Though the internal temperature was higher than the boiling point under atmospheric pressure,but the local pressure inside lumber was higher than atmospheric pressure. The final MC of center layer of poplar lumber with three different heating platens' temperatures was higher than 50%,which proved the guess of liquid state in the center layer of lumber. 3) Moisture state analysis inside the poplar lumber during the opening period of heating platens showed that flashing phenomena occurred to unsaturated water in poplar lumber with the pressure's sudden dropping to the atmospheric pressure,and that liquid water inside lumber was changed into vapor and vapor was discharged from surface of lumber,and these resulted in the decreasing of MC of lumber. Conclusion: In the closing phase of heating platens,vapor pressure inside wood,which was resulted from the water evaporating between surface and evaporation front,was higher than the atmospheric pressure. Moisture state distribution of superheated vapor (surface) -saturated vapor (evaporation front) -liquid water (center) was formed from surface to center of lumber along the thickness orientation. In the opening phase of heating platens,vapor driven by pressure difference between the inside and outside of lumber was discharged to environment,and resulted in the decreasing of wood MC,and these are the main ways of moisture migration in wood with high MC in platen drying.

Key words: platen drying, heating platens' temperature, poplar lumber, moisture state, temperature, pressure